In the bottling industry, prior art plant for making plastic bottles comprises: a blow-moulding wheel mounted in such a way as to rotate about its longitudinal axis and equipped with a plurality of blow-moulding units, each of which is rotationally fed by the blow-moulding wheel about this axis and comprises two cavities for blow-moulding respective bottles; an oven for thermally conditioning the parisons; a feed device for advancing the parisons through the oven according to a first, predetermined spacing; and a transfer device for transferring the parisons from the feed device to the blow-moulding cavities.
Since the first predetermined spacing is normally smaller than the distance between the longitudinal axes of the moulding cavities of one blow-moulding unit, the transfer device comprises a first variable spacing transfer wheel designed to pick up each parison from the feed device and space them according to this distance, and a second transfer wheel designed to transfer pairs of parisons from the first transfer wheel to the respective moulding cavities.
Alternatively, the prior art in this industry also teaches the use of plant comprising two ovens for thermally conditioning the parisons; two feed devices for advancing the parisons through the ovens; and one transfer wheel equipped with a plurality of pick-up and transporting units each comprising two pick-up and transporting members whose respective longitudinal axes are spaced at a distance from each other that is equal to the distance between the longitudinal axes of the moulding cavities of one moulding unit, which are designed to pick up one parison each from a respective feed device and to transfer a pair of parisons to the respective moulding cavities.
Plants of the type described above have several disadvantages, due mainly to the fact that the presence of two transfer wheels or, alternatively, two ovens and two feed devices, makes these plants relatively complex and expensive.
Also known in the prior art, from patent document DE19906309, is a system of transferring parisons from the oven to the blow-moulding machine. The system comprises associating the parisons in pairs to gripping elements which are in turn removably associated with both the conveyor of the oven and with the transfer star wheel interposed between the oven and the blow-moulding machine. Thus, the parisons are always associated with the gripping elements which move from the oven to the transfer star wheel carrying the parisons along with them. This enables pairs of parisons to be kept at a desired constant distance from each other (set by the geometry of the gripping element) during transfer.
This solution, too, however, is not free of disadvantages.
That is because the presence of gripping elements that move along the plant involves a complication in the plant itself and increases the risk of faults and malfunctioning. Moreover, the presence of gripping elements interposed between the parisons and the conveyor of the oven makes it difficult to optimize parison spacing inside the oven.